1. Field of the Invention
The present invention relates to an aspherical spectacle (viewing) lens whose power (back vertex power) is a negative value, and more precisely it relates to a negative aspherical spectacle lens having a small thickness.
2. Description of Related Art
In a spectacle lens consisting of a spherical lens, once a surface power of the lens is determined, an allowable base curve which defines the lens profile is automatically determined in accordance with the degree of the lens to restrict aberration of the lens within a tolerance. As the power increases, the base curve becomes steep, resulting in a considerable thickness (total thickness and edge thickness) of the lens. Consequently, it is impossible to realize a thin spherical spectacle lens whose aberration is within the tolerance.
To solve the problem of the spherical spectacle lens discussed above, it is known to use an aspherical lens instead of the spherical lens to realize a thinner lens while preserving the same power. The spectacle lens, consisting of an aspherical lens having an aspherical surface located on the object side, can be made thinner than the spherical spectacle lens without increasing the aberration.
Nevertheless, in conventional aspherical spectacle lenses, it has been proposed that the base curve of the lens profile to be corrected by the aspherical surface be only slightly less steep than the base curve for the spherical lens. The inventors of the present invention have found that one cannot expect to remarkably reduce the lens thickness with such a base curve for the conventional aspherical spectacle lenses.